Bluetooth wireless technology is an emerging communication solution that allows users to make wireless and instant connections between various communication devices, such as mobile phones and desktop and notebook computers. Because Bluetooth uses radio transmissions, transfer of both voice and data can occur in real time. This sophisticated mode of transmission adopted in the Bluetooth specification ensures protection from interference and security of data.
The Bluetooth radio is designed to operate in a globally available frequency band, ensuring communication compatibility worldwide. The Bluetooth specification has two power levels defined; a lower power level that covers a personal area within a room, and a higher power level that can cover a larger area, such as a home. Software controls an identity coding built into each radio to ensure that only those units preset by their owners can communicate with one another.
Bluetooth wireless technologies support both point-to-point and point-to-multipoint connections. With the current specification, up to seven “slave” devices can be set to communicate with a “master” radio in one device. Several of these so-called “piconets” can be established and linked together in ad hoc “scatternets” to allow communication among continually flexible configurations. All devices in the same piconet have priority synchronization, but other devices can be set to enter at any time. This topology can best be described as a flexible, multiple piconet structure.
Although Bluetooth presents an extremely flexible and desirable communication architecture for computer vendors and vendors of other personal digital assistants and the like, there are several challenges associated with implementing Bluetooth-compatible devices. For example, the specifications governing the implementations of Bluetooth-compatible transmitters and receivers (see, Specification of the Bluetooth System, vol. 1.0b, Dec. 1, 1999, published by Bluetooth Special Interest Group and available via the World Wide Web at www.bluetooth.com) indicate that such devices must be capable of operating using Gaussian Frequency Shift Keying (GFSK) with a BT product of approximately 0.5 and a deviation index or modulation index (h) of approximately 0.32. In this scheme, a binary “1” is represented by a positive frequency deviation (from the carrier frequency) and a binary “0” is represented by a negative frequency deviation.
The “BT” product is the product (i.e., a mathematical multiplication operation) of the occupied bandwidth of a communication signal and the bit period thereof. It is used by engineers and others in the relevant art as a shorthand expression for communicating information regarding the effective band limiting of a transmitted signal. With Bluetooth, the bit period (T), which is an indication of the keying rate, is specified as 1 MHz. Thus, the available bandwidth for a transmitted signal to occupy (B) is 0.5 MHz.
The deviation index (h) is a measure of the difference in frequency for an FSK modulation scheme (as is used by Bluetooth radios) between different bits. That is, the difference in the modulation frequency for transmission of a logical “1” versus a logical “go”. Since the modulation frequency (or keying rate) is specified as 1 MHz and h=0.32, this gives a maximum deviation frequency fD=(0.32×1)/2 MHz=160 kHz.
This small deviation index presents a problem. One cycle of a 160 kHz sine wave has a period of approximately 6.25 μsec. But the keying rate in Bluetooth is 1 MHz (i.e., a period of 1 μsec), so that even before a complete cycle of a bit can be modulated onto the carrier during transmission, the time for transmitting the next bit has already arrived. Thus, at most only approximately one sixth of a cycle of the data signal will be available at the receiver for decoding. For this reason a decoding solution for a low deviation index transmission scheme is needed.
In one embodiment, a wireless receiver, e.g., a Bluetooth-compatible receiver or receive chain in a Bluetooth-compatible transceiver, includes a discriminator unit and a timing recovery unit. The output of the discriminator unit is provided as an input to the timing recovery unit, which timing recovery unit is configured to align a free-running clock of the receiver with a received signal to extract received data therefrom.
These and other embodiments of the present invention are more fully described below and illustrated in the drawings.